Enhancement: Add --last-ver command line option for mixer init

[gtkramer]

DevOps needs to know about the internals of mixer in order to keep it's operations running when things go wrong. If a build is bad and cannot be used in the update chain for swupd for whatever reason, DevOps currently writes to the LAST_VER file manually with an echo with the proper build that needs to be here. In favor of passing more options at the command line, this LAST_VER should be another option that can be passed during mixer init, just as format can, so that mixer.state is always created correctly prior to starting the mix.

[gtkramer]

Today, this prints the following messages:

$ mixer build update --offline --config /home/clrbuilder/projects/clr-release-tools/chroots/builder.conf --min-version 29750 --format 28 --no-publish --no-signing --native
Warning: Using PREVIOUS_MIX_VERSION default value

It would be beneficial to have a command line option that defined this value, removing the need to warn how mixer.state is managed.

[gtkramer]

Would be obsoleted with a CLI design similar to #627